1. Field of the Invention
The present invention relates to reflection sheets for a backlight unit which reduce loss of rays of light, and backlight units in which this reflection sheet is used.
2. Description of the Related Art
Liquid crystal equipments in widespread use have been in a backlight system where light emission is executed by irradiating onto a liquid crystal layer from the back face. In such a type of equipment, a backlight unit which is edge light type, immediate beneath type or the like is provided to an under face side of the liquid crystal layer Such a backlight unit 20 of an edge light type is generally equipped with a rod-shaped lamp 21 for use as a light source, an optical waveguide plate 23 having a square plate shape disposed so that the edge thereof abuts along the lamp 21, multiple optical sheets 22 overlaid to the front face side of the optical waveguide plate 23, and a reflection sheet 26 overlaid to the back face side of the waveguide plate 23, as shown in FIG. 3. Each of the optical sheets 22 has specified optical properties such as refraction, diffusion and the like. Specifically, included are a light diffusion sheet 24 provided on the front face side of the optical waveguide plate 23, a prism sheet 25 provided on the front face side of the light diffusion sheet 24, and the like.
Referring to functions of the backlight unit 20, rays of incident light from the lamp 21 to the optical waveguide plate 23 are first reflected on reflection dots (not shown in the Figure) of the back face of the waveguide plate 23 and on each of the side faces, and exit from the front face of the waveguide plate 23. The rays of light that exited from the waveguide plate 23 enter into the light diffusion sheet 24, then diffuse and exit from the front face of the light diffusion sheet 24. Thereafter, the rays of light exited from the light diffusion sheet 24 enter into a prism sheet 25, and exit as rays having a distribution representing a peak in a direction along a substantially normal line through a prism part 25a formed on the front face of the prism sheet 25. Accordingly, the rays of light exited from the lamp 21 are diffused by the light diffusion sheet 24, while being refracted by the prism sheet 25 so that they represent a peak in a direction along the substantially normal line, and illuminate the entire face of the upper liquid crystal layer although not shown in the Figure.
Meanwhile, a reflection sheet 26 is provided to the back face side of the optical waveguide plate 23 for the purpose of: attempting the improvement of brightness through reflecting rays of light that are radiated from the back face of the optical waveguide plate 23 to the front face side in an extent that glare does not result thereby (reflection property); and preventing instruments, frames and the like that are provided to the back face side of the backlight unit 20 from becoming visible (concealing property). Examples of such a reflection sheet 26 which may be conventionally used include: (a) sheets made of a synthetic resin including a white pigment dispersed therein; (b) sheets made of a synthetic resin including voids (bubbles) dispersed therein for light scattering; (c) sheets having a surface formed into a mat shape; and the like.
Because the conventional reflection sheets 26 described above exhibit concealing property which is not that eminent, reflectance may be too high on some parts such as a frame that supports the backlight unit 20, thereby causing lack in uniformity of the brightness.
In order to prevent such disadvantages, printing for decreasing the reflectance on the parts such as frames described above which are responsible for lack in uniformity of the brightness, or printing for improving the concealing property to the back face of the reflection sheet 26 may be required, and thus the manufacturing process becomes complicated in connection with such a printing operation.